tow vehicle tire air pressure/load quandary

Seems to me you're going at it backward. I'd take the rig, fully loaded and get at least an axle weight for each axle, preferably individual wheel weights. Then when I buy new tires, go into the tire manufacturers weight/pressure rating and pick the proper pressure for that axle loading. The Chevy recommended pressures are for the vehicle with driver and one or more passengers, running the OEM tires, not loading it as you are doing. Running a tire above max rated pressure shown on the sidewall is never a good idea, but OEM tires are not typically put on a vehicle to carry the loads you want to carry.